The 11q23 gene MLL (Mixed Lineage Leukemia) is involved in a number of acute leukemia associated interchromosomal translocations. In all of the translocations characterized at a molecular level MLL has formed a chimerical gene encoding an in frame fusion transcript with a partner gene. The diseases seen with these translocations include Acute Leukemia in patients who have been treated with topoisomerase II inhibitor chemotherapy (secondary leukemia). In this work we will study the role the partner gene plays in leukemogenesis and the mechanism which generates the MLL/11q23 translocations. As a prototypic MLL fusion partner we will study the 4q21 MLL fusion partner AF4. We have already cloned the murine homologue of AF4 and preliminarily shown it to be a nuclear protein. In the studies proposed here we will determine the normal biochemical function of murine AF4 and its structure-function relationships. To do this we will generate antibodies to the murine AF4 protein and confirm its subcellular localization, identify proteins it interacts with, and determine its phosphorylation status. We will analyze the structure-function relations which determine the AF4 protein's subcellular localization and stability. We will also determine if murine AF4 binds nucleic acids in a sequence specific manner. In order to gain insight into the mechanism of translocation, we have cloned the der(11) breakpoints from four patients with MLL/11q23 secondary acute myelogenous leukemia and one with MLL/11q23 secondary acute lymphoblastic leukemia. In the work proposed in this application we will determine if specific proteins interact with the DNA sequences from these breakpoints. To understand the topoisomerase II-induced DNA rearrangements of lymphoblastoid cell lines derived from patients with Bloom's syndrome, Fanconi Anemia, Atazia Telangiectasia, Xeroderma Pigmentosum, Cockayne's Syndrome, and MLL/11q23 secondary leukemia. From this work we should gain insight both into the mechanisms of leukemogenesis and MLL translocations.